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"In order to properly evaluate the 'fairness' of a tax system, you have to compare tax burdens to income levels, or ability to pay," says Diane Lim Rogers, chief economist at the Concord Coalition, a group that champions fiscal responsibility. By contrast, she says via email, a focus on the "share of taxes paid" can be tricky or misleading.

Ms. Rogers says that when President Bush cut taxes, the share of income taxes paid by the rich went up, but that doesn't mean the tax code had become more progressive. "The rich were paying a larger share of a much smaller pie," she says. Even so, the rich "enjoyed the largest relative (not just absolute) decline in tax burdens" she says, so progressivity actually fell, when tax burdens are measured by income level.